A method and device for high speed spectroscopic constituent verification. The method includes the steps of illuminating a sample with broadband light and measuring two preselected wavelengths of reflected light: a first narrow-range of wavelengths .lambda.1 that is not significantly absorbed by the constituent of interest, and a second narrow-range of wavelengths .lambda.2 that is substantially absorbed by the constituent of interest. Given the two measurements of reflection, upper and lower thresholds are determined (the latter based on a percentage of the measured baseline reflected light of wavelengths .lambda.1). Finally, the presence of the constituent of interest is indicated if the measure of the reflected discriminant wavelength .lambda.1 is within te upper and lower threshold measures of reflected baseline light .lambda.1. The compact device that implements the above-described method includes a base unit housing a near-IR discriminator circuit with a pair of selective light sensors, and a light source. A sensor unit connects to the base unit. The sensor unit includes a flexible neck with a connecting block attached at one end for mating with the base unit, and a hood assembly attached at the other end for enclosing a light collecting and transmitting lens. The optics are connected by an optical fiber bundle inside the flexible neck.